Systems and methods for sync mark detection using correlation

ABSTRACT

Systems and methods for detecting and designing enhanced disk sync marks using correlation detection are disclosed. The enhanced sync marks provide better noise immunity and higher detection rates over traditional Viterbi-based detection schemes even with a shorter sync mark length. The disk sync mark may provide optimal noise immunity for a particular target polynomial or a plurality of common target polynomials. The minimum Euclidean distance between a candidate sync mark and a plurality of right-shifted versions of the candidate sync mark is computed and compared with other candidate sync marks. The sync mark with the largest minimum Euclidean distance is then selected as the optimal mark. Systems and methods are also disclosed for detecting and designing a disk sync mark using correlation detection when the polarity of the disk is unknown or time-varying.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a non-provisional patent application claiming thebenefit of U.S. Provisional Patent Application No. 60/811,665, filedJun. 7, 2006, which is hereby incorporated by reference herein in itsentirety.

BACKGROUND OF THE INVENTION

This invention relates generally to magnetic recording channels and,more particularly, to magnetic recording channels with improved syncmark design and detection using correlation.

Data is stored on contemporary magnetic hard disk drives in concentrictracks around the recording surface or surfaces of the disk or disks.Each track may be divided into a number of addressable sectors, witheach sector including a preamble, sync mark, and user data.

The preamble may, for example, contain a portion that enables the readchannel to adjust its gain and allow a phase locked loop (PLL) toachieve bit synchronization. The preamble may also include a DC eraseportion in which there are no logical transitions (e.g., anuninterrupted string of zeros) for a specified length. Since a string ofbits with no logical transitions may be illegal everywhere else on thedisk, the DC erase portion of the preamble may uniquely identify thisportion as being part of the sector preamble.

The sync mark may follow the preamble on the disk and may be designed tobe of any suitable length. Since the next bit of data after the syncmark starts the user data, it is crucial that the sync mark be easilydetected and detected extremely reliably. Current sync mark designs mayimpose several restraints on the sync mark to increase detectionprobability, including, for example, no T-spaced transitions and notransitions on byte boundaries, to reduce or eliminate most-likely errorevents.

As data densities and data rates continue to increase, however, morenoise is inevitably introduced into magnetic data channels. For example,in order to increase the areal densities of magnetic media, many mediamanufacturers are using perpendicular recording. Unlike traditionallongitudinal recording, where the magnetization is lying in the plane ofthe magnetic medium, with perpendicular recording the media grains areoriented in the depth of the magnetic medium with their magnetizationpointing either up or down, perpendicular to the plane of the disk.Using perpendicular recording, manufacturers have exceeded magneticrecording densities of 100 Gbits per square inch, and densities of 1Terabit per square inch are feasible.

As storage densities increase, the signal processing and detection ofmagnetic channels becomes more difficult. Sources of distortion,including media noise, electronics and head noise, inter-trackinterference, thermal asperity, partial erasure, and dropouts, arebecoming more pronounced. Particularly troublesome are signal-dependenttypes of noise, such as transition jitter, because these types of noiseare quickly becoming the dominant sources of detection errors.

Because bit error rates (BER) in these new magnetic recording channelsare increasing, a new sync mark design and detection scheme is needed tomore reliably detect disk sync marks. Traditional sync mark detectionschemes using Viterbi detection may be inadequate to support these highdata rate channels. If a disk sync mark is not reliably detected on thefirst attempt, the disk spindle motor typically must drive the diskcompletely around again and attempt redetection of the sync mark. Thisreduces drive data rates and cripples overall system performance.

Accordingly, it is desirable to provide systems and methods for enhancedsync mark detection using correlation. The enhanced sync mark design anddetection scheme may provide a gain of several dB over traditional syncmark designs.

SUMMARY OF THE INVENTION

In accordance with principles of the present invention, an improved syncmark design and detection scheme for magnetic data channels is provided.One improvement of the sync mark detection scheme is the use ofcorrelation detection instead of Viterbi-based detection in detectingthe improved sync mark on the hard disk.

To determine the optimal disk sync mark for use with correlationdetection, an exhaustive search may be performed on the bit space ofpotential, bit patterns of a desired length. First, an initial patternis selected for the sync mark. Then, a plurality of right bit-shiftedsync mark patterns are defined, with the disk preamble shifted into thehigher-order bit positions of the shifted patterns. The minimumEuclidean distance is then computed between the selected sync markpattern and all the shifted versions of the selected sync mark pattern.This process may be repeated until all potential bit patterns in the bitspace have been tested. The bit pattern with the largest minimumEuclidean distance between the bit pattern and all the right-shiftedversions of the bit pattern is selected as the optimal disk sync mark.

In some embodiments, to reduce the bit space of potential sync markpatterns, adjacent bits of the selected sync mark pattern are paired up(i.e., the first and second bits are paired, the third and forth bitsare paired, and so on). A restriction is then imposed on all thepotential sync mark patterns so that these paired up bits are requiredto take the same value. This restriction may decrease the bit space ofthe sync mark pattern search from 2^(N) potential patterns to 2^(N/2)potential patterns, where N is the length of the sync mark. Imposingthis restriction may exponentially reduce the time needed to completethe sync mark search while still yielding a near-optimal pattern for thesync mark.

Other assumptions may also be made, in some embodiments, to reduce thecomplexity, or to increase the applicability, of the sync mark search.For example, in magnetic storage systems the recording channel isgenerally shaped to a specific target polynomial or target response. Thesync mark may be selected assuming a particular target (e.g., [4, 7, 1])or an exhaustive search using all common targets may be performed toyield the overall best general-purpose sync mark.

In some embodiments, especially when the polarity of the system isunknown (or time-varying), a plurality of right bit-shifted versions ofthe selected sync mark pattern and a plurality of right bit-shiftedversions of the sign-flipped sync mark pattern may be defined. Theminimum Euclidean distance is then computed between the selected syncmark pattern and all the shifted versions of both the selected sync markpattern and the sign-flipped sync mark pattern. After the minimumEuclidean distance is computed for all potential bit patterns, the bitpattern with the largest minimum Euclidean distance between the bitpattern and all the shifted versions of both the bit pattern and thesign-flipped bit pattern is selected as the optimal disk sync mark.Various assumptions and restrictions (e.g., as described above) may alsobe imposed on the sync mark patterns in these embodiments as well.

To detect the new sync mark, correlation detection is used instead ofViterbi-based detection. Detection timing is established by the sectorpreamble, and the detection may be performed on the same period as theperiod of the preamble (e.g., every 4 bit patterns, or 4 T). Inembodiments where the polarity is unknown or time-varying, the preamblemay be used to establish two timing phases (a first phase and a second,opposite phase shifted by 180 degrees) and correlation detection may beperformed more frequently (e.g., twice as frequently, or every 2 T).

In some embodiments, an apparatus for detecting a disk sync mark isprovided including means for reading a plurality of sector preamble bitsand means for establishing at least one timing phase based on the sectorpreamble bit read. Correlation detection circuitry means is provided fordetecting the disk sync mark, the detection performed by the correlationdetection circuitry means at a period determined by the at least onetiming phase.

Further features of the invention, its nature and various advantages,will become more apparent from the accompanying drawings and thefollowing detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of an illustrative disk platter inaccordance with one embodiment of the invention;

FIG. 2 is a diagram of an illustrative preamble, sync mark, and userdata structure in accordance with one embodiment of the invention;

FIG. 3 shows an illustrative sinusoidal readback waveform correspondingto a sector preamble read in accordance with one embodiment of theinvention;

FIG. 4 is an illustrative sliding window for defining right bit-shiftedversions of a potential sync mark in accordance with one embodiment ofthe invention;

FIG. 5 shows three illustrative sync mark read windows in accordancewith one embodiment of the invention;

FIG. 6 is an illustrative process for selecting the optimal sync markpattern when the polarity is fixed or known in accordance with oneembodiment of the invention;

FIG. 7 is an illustrative process for selecting the optimal sync markpattern when the polarity is time-varying or unknown in accordance withone embodiment of the invention;

FIG. 8 is an illustrative process for detecting a disk sync mark usingcorrelation detection in accordance with one embodiment of theinvention; and

FIG. 9 is an illustrative hard disk drive employing the sync mark designand/or detection of the present invention.

DETAILED DESCRIPTION

Embodiments of the present invention relate to systems and methods fordetecting disk sync marks using correlation detection. The presentinvention also provides an improved sync mark design for use withcorrelation detection.

The improved sync mark design and detection scheme using correlationdetection may be implemented in any magnetic hard disk drive, includingdrives implementing perpendicular recording. For example, FIG. 1 showsillustrative hard disk 100 in accordance with one embodiment of thepresent invention. Disk platter 102 may contain numerous concentric datatracks, such as track 104. These tracks may be divided into sectors,with each sector including sector preamble 106, sector sync mark 108,and user data 110.

FIG. 2 shows illustrative sector data 200 in accordance with oneembodiment of the present invention. Sector data 200 includes preambleportion 202, which may be used to establish timing and bitsynchronization. In some embodiments, preamble portion 202 includes 120bits, but any suitable number of bits may be used in other embodiments.The preamble pattern may include a repeating “1100” pattern portion andan optional DC erase portion.

After preamble portion 202, sector data 200 may include a sector syncmark. Sync mark 204 may include any suitable number of bits. In someembodiments, the length of sync mark 204 is some multiple of thepreamble period. One example uses a 4 T-period repeating “1100” patterndescribed above as the sector preamble. Then sync mark 204 may be 4Nbits long, where N is any positive integer. The bit following sync mark204 begins user data portion 206.

As described above, in typical hard drive operation it is crucial thatthe sector sync mark be detected easily and reliably so that the startof the user data may be readily identified. If detection of the syncmark fails, the disk may have to rotate around to attempt detection ofsync mark again, which is inefficient and highly undesirable. This canadversely effect data rates and overall system performance.

FIG. 3 shows illustrative readback graph 300 in accordance with oneembodiment of the present invention. In the depicted embodiment,readback waveform 302 is the readback waveform for the sector preambleand is substantially sinusoidal with a period of 4 T. It is to beclearly understood, however, that the period of the sector preamblereadback waveform may be larger or smaller than 4 T in otherembodiments.

To determine the optimal sync mark to be used for correlation detection,an exhaustive search of all potential sync mark bit patterns may beperformed in some embodiments. For example, if a 4N-bit sync mark isdesired, then an exhaustive search may be performed of all potential4N-bit patterns for use as the sector sync mark. The optimal sync markwill be the candidate with the greatest immunity to noise, as measuredby the Euclidean distance between the pattern and all 4-bitright-shifted bit patterns, as described in more detail with regard toFIGS. 4-6, below.

As shown in FIG. 4, a sliding window may be used to define Nright-shifted versions of each sync mark to be tested. The lower orderbits are shifted out, while the sector preamble is shifted into thehigher order bits. Although, in the example of FIG. 4, four bits areshifted out and four preamble bits are shifted into the window, this isdone for clarity only and not by way of limitation. Preferably, thenumber of bits shifted in and out is the number of bits in one period ofthe sector preamble. This is done because correlation detection may beperformed based on the preamble period, as described in more detailbelow.

FIG. 5 shows illustrative sector data 500. As shown in FIG. 2, sectordata 500 may include preamble portion 504, sync mark portion 504, anduser data portion 506. Because, in some embodiments, detection timing isestablished by the sector preamble, correlation detection may beperformed every preamble period (e.g., 4 T). For example, onecorrelation decision may be based on read window Δ₁. The next detectiondecisions may occur at read windows Δ₂ and Δ₃, as more bits are read inthe direction of disk rotation toward the actual sync mark. As can beseen from the example of FIG. 5, eventually the correlation detectionread window will encompass the actual sync mark, and the sync mark maybe declared detected.

To determine the optimal sync mark pattern, the minimum Euclideandistance, D_(min), between a sync mark candidate pattern and allright-shifted versions of the sync mark candidate pattern may becomputed in accordance with:D _(min)(SM ₀)=minE{SM ₁ , . . . , SM _(N)}  (EQ 1)where SM₀ is the sync mark candidate to be tested, E is the Euclideandistance function, and SM₁ through SM_(N) are right-shifted versions ofSM₀. Preferably, the right-shifted versions are shifted the number ofbits of the preamble period (i.e., the period of the correlationdetection). In this way, D_(min) represents a noise immunity metric forthe sync mark candidate, SM₀, that may be used to compare the noiseimmunity of SM₀ with the noise immunity of other potential sync markcandidates.

In a similar manner, an exhaustive search is performed over allpotential sync mark candidates for the optimal candidate pattern. Thus,a search is performed for the sync mark candidate with the greatestimmunity to noise (i.e., the largest minimum Euclidean distance betweenthe candidate pattern and all shifted versions of the candidate pattern)in accordance with:D _(max)=max{D _(min)(SM ₀)}  (EQ 2)for all potential candidate patterns, SM₀. For example, if the desiredsync mark is 4N bits in length, an exhaustive search for all SM₀ in[0˜2^(4N)] may be performed. The sync mark candidate with the greatestimmunity to noise metric is the optimal candidate and may be used as thedisk sync mark.

To reduce the complexity of the search, in some embodiments, one or moresimple restrictions are imposed on the form of potential sync markcandidates. These restrictions do not significantly effect the sync markcandidate's immunity to noise, but may help drastically improve theoptimal sync mark pattern search time and search complexity. Forexample, every two adjacent bits (e.g., bits 1 and 2, bits 3 and 4, bits5 and 6, etc.) of the sync mark candidate may be paired and forced totake the same value. For a sync mark length of N, this can reduce thecomplexity of the search from 2^(N) to 2^(N/2). By facilitating thesearch in this way, the optimal sync mark may be found in less time, andthe search may consume less system resources.

In some embodiments, the sync mark search is also performed acrossmultiple target channels. For example, the pattern with the greatestnoise immunity using EQ 2, above, for targets [4,7,1,0], [5,6,0,−1],[5,4,−2,−1], and [5,5,−2,0] may be computed. The best overall sync markfor all common targets may then be used as the disk sync mark. Theaforementioned targets are merely illustrative. Any number and form oftarget polynomials may be included in the search for the optimal syncmark, if desired.

The optimal sync mark pattern for correlation detection yields aperformance gain of several dB over traditional sync mark designs evenwith a much shorter sync mark length. For example, with the enhanceddesign described above, a 12-bit sync mark has almost a 4 dB gain innoise immunity over a traditional 36-bit sync mark design. The use of ashorter sync mark may increase the storage capacity of the disk andincrease data rates as well. More importantly, the increase in noiseimmunity results in a more reliable sync mark detection.

In some embodiments, in order to remove DC offset, a detected sample ispassed through a 1-D or 1-D² block (or any other filter or circuitryoperable to remove DC offset). Simulated results with signal phase errorof 0.1 T and signal asymmetry of −30 percent still yielded over a 2 dBgain in noise immunity over traditional sync mark designs usingViterbi-based detection. This performance gain was seen even with a muchshorter sync mark (i.e., a 12-bit mark as opposed to a standard 36-bitor longer mark).

In some embodiments, the polarity of the disk may be unknown. To detecta sync mark on a drive whose polarity is unknown, correlation detectionmay be performed at every P/2 bit periods, where P is the preambleperiod. The preamble may be used to define two phases, a normal phaseand an opposite phase inverted by 180 degrees. Right-shifted versions ofthe sync mark and the sign-flipped sync mark may then be defined. On thenormal phase defined by the sector preamble, the correlation detectionwindow is looking for the sync mark, but may instead see a P-bitright-shifted version of the sync mark or a (P+P/2)-bit right-shiftedversion of the sign-flipped sync mark. Similarly, on the opposite phasedefined by the sector preamble, the correlation detection window islooking for the sign-flipped sync mark, but may instead see a P-bitright-shifted version of the sign-flipped sync mark or a (P+P/2)-bitright-shifted version of the sync mark.

Accordingly, the optimal sync mark design when the polarity is not knownor time-varying may be the sync mark pattern with the maximum Euclideandistance between the sync mark pattern and any of the P-bitright-shifted versions of the sync mark and the (P+P/2)-bitright-shifted versions of the sign-flipped sync mark. The bits shiftedinto the right-shifted versions of the sync mark pattern andsign-flipped sync mark pattern are the preamble bits and sign-flippedpreamble bits, respectively.

The sync mark pattern that maximizes the minimum Euclidean distancebetween the sync mark pattern and any of the P-bit right-shiftedversions of the sync mark and the (P+P/2)-bit right-shifted versions ofthe sign-flipped sync mark also maximizes the Euclidean distance betweenthe signed-flipped sync mark pattern and both the P-bit right-shiftedversions of the sign-flipped sync mark and the (P+P/2)-bit right-shiftedversions of the sync mark. Even with unknown polarity, the optimal syncmark using correlation detection yields a noise immunity gain of severaldB over traditional sync marks designs of much longer length.

FIG. 6 shows illustrative process 600 for selecting an optimal sync markfor correlation detection. At step 602, a 4N-bit pattern is initiallyselected as a sync mark candidate. For example, the all ones pattern,all zeros pattern, or the optimal pattern for use with Viterbi-baseddetection schemes may be initially selected at step 602. At step 604, Nshifted versions of the selected pattern are defined, each shiftedversion right-shifted by 4j bits, where j is an integer from 1 to N. Theappropriate number of sector preamble bits may be shifted into theshifted versions of the selected pattern.

At step 606, the minimum Euclidean distance between the selected patternand all the defined shifted versions is computed. At step 608, adetermination is made whether all potential sync mark patterns have beentested. For example, in some embodiments, the entire bit space (i.e.,2^(4N) patterns) are tested. In other embodiments, adjacent pairs ofbits (i.e., bits 1 and 2, bits 3 and 4, bits 5 and 6, etc.) are forcedto take the same value, facilitating the computation by reducing the bitspace to search. If not all of the potential bit patterns have beentested, then process 600 returns to step 602 by selecting a new 4N-bitpattern as a sync mark candidate.

Once, at step 608, a determination is made that all the potential syncmark patterns have been tested, at step 610 the sync mark candidate withthe largest minimum Euclidean distance between the sync mark candidateand all the shifted versions of the sync mark candidate is selected asthe optimal sync mark. This sync mark may then be written to the disk.

In some embodiments, the exhaustive sync mark search is performed for asingle target polynomial. In other embodiments, the optimal sync markfor several common target polynomials is searched and selected as theoptimal sync mark.

In practice, one or more steps shown in illustrative process 600 may becombined with other steps, performed in any suitable order, performed inparallel (e.g., simultaneously or substantially simultaneously) orremoved.

FIG. 7 shows illustrative process 700 for selecting an optimal sync markfor correlation detection when the disk polarity is unknown ortime-varying. At step 702, a 4N-bit pattern is initially selected as async mark candidate. For example, the all ones pattern, all zerospattern, or the optimal pattern for use with Viterbi-based detectionschemes may be initially selected at step 702. At step 704, N shiftedversions of the selected pattern are defined, each shifted versionright-shifted by 4j bits, where j is an integer from 1 to N. Theappropriate number of sector preamble bits are shifted into the shiftedversions of the selected pattern. At step 706, N shifted versions of thesign-flipped selected pattern are defined, each shifted versionright-shifted by 4j+2 bits, where j is an integer from 0 to N−1.

At step 708, the minimum Euclidean distance between the selected patternand all the defined shifted versions (and sign-flipped shifted versions)is computed. At step 710, a determination is made whether all potentialsync mark patterns have been tested. For example, in some embodiments,the entire bit space (i.e., 2^(4N) patterns) are tested. In otherembodiments, adjacent pairs of bits are forced to take the same value,facilitating the computation by reducing the bit space. If not all ofthe potential bit patterns have been tested, then process 700 returns tostep 702 by selecting a new 4N-bit pattern as a sync mark candidate.

Once, at step 710, a determination is made that all the potential syncmark patterns have been tested, at step 712 the sync mark candidate withthe largest minimum Euclidean distance between the sync mark candidateand all the shifted versions of the sync mark candidate (andsign-flipped sync mark candidate) is selected as the optimal sync mark.

In practice, one or more steps shown in illustrative process 700 may becombined with other steps, performed in any suitable order, performed inparallel (e.g., simultaneously or substantially simultaneously) orremoved.

FIG. 8 shows illustrative process 800 for detecting the sync mark of thepresent invention using correlation detection in accordance with oneembodiment of the invention. At step 802, the sector preamble is read.For example, in some embodiments, the sector preamble contains a 120-bitrepeating “1100” pattern. In other embodiments, longer or shorterpreambles may be used. Timing parameters may then be established fromthe sector preamble read. For example, a PLL may be locked to the phaseof the sector preamble. One or more timing phases may then beestablished at step 804. For example, as described above, two phases (anormal phase and an opposite phase shifted by 180 degrees) may beestablished from the sector preamble if the polarity of the disk isunknown or time-varying. As another example, a single timing phase maybe established from the sector preamble read if the polarity is known.If a determination is made at step 806 that the polarity is known,correlation detection may be performed every P clock cycles at step 810,where P is the period of the timing phase or phases established at step804. If, at step 806, the polarity is not known, then correlationdetection may be performed every P/2 clock cycles at step 808.

In some embodiments, in order to perform correlation detection, aparticular target may be assumed. The target output sequence may becalculated for the expected sync mark pattern, S₀. The sign-flippedversion of S₀, ˜S₀, may then be defined, as well as the one or moreright-shifted versions of S₀ and/or ˜S₀. For example, in oneillustrative embodiment, the 4i-T right-shifted version and the 4i+2-Tright-shifted version of S₀ is defined. Similarly, the 4i-Tright-shifted version and the 4i+2-T right-shifted version of ˜S₀ mayalso be defined. These right-shifted versions of S₀ and ˜S₀ are merelyillustrative. Any other right-shifted versions of S₀ and ˜S₀ mayadditionally or alternatively be defined. Based on the sector preambleread, a timing loop may be locked to two phases, Θ₀ and Θ₁. On Θ₀, thecorrelation detection circuitry may look for the expected sync markpattern S₀. However, instead of seeing S₀, one of the right-shiftedversions of S₀ or ˜S₀ may be seen by the correlation detectioncircuitry.

On Θ₀, the Euclidean distance between the received sequence, R, and S₀may be computed. If the Euclidean distance is less than a target minimumEuclidean distance, the correlation detection circuitry may signal thatthe sync mark has been found and the polarity of the disk is positive.If the Euclidean distance between the received sequence R and S₀ is notless than the target minimum Euclidean distance, the correlationdetection circuitry may attempt detection on Θ₁, which is the oppositephase of Θ₀. On Θ₁, the correlation detection circuitry may look for thesign-flipped version of the expected sync mark pattern, or ˜S₀. However,instead of seeing ˜S₀, one of the right-shifted versions of S₀ or ˜S₀may be seen by the correlation detection circuitry.

On Θ₁, the Euclidean distance between the received sequence, R, and ˜S₀may be computed. If the Euclidean distance is less than a target minimumEuclidean distance, the correlation detection circuitry may signal thatthe sync mark has been found and the polarity of the disk is negative.If the Euclidean distance between the received sequence, R, and S₀ isnot less than the target minimum Euclidean distance, the correlationdetection circuitry can attempt detection on Θ₀ once again for the nextread window. This process may continue until the sync mark is detected(or an error condition, timeout, or end of sector condition occurs).

There are several advantages of performing sync mark correlationdetection every P or P/2 clock cycles, where P is the period of thetiming phase or phases established at step 804, instead of every clockcycle. Hardware requirements may be significantly reduced, and thedetection pattern is given more time to settle. Moreover, a sync markpattern with greater immunity to noise may be selected as the disk syncmark, as described above.

Correlation detection may continue until the sector sync mark isdeclared detected at step 812. After sync mark detection, the next bitof data may begin the user data. After detection of the sync mark, atstep 814 the user data read may begin.

In practice, one or more steps shown in illustrative process 800 may becombined with other steps, performed in any suitable order, performed inparallel (e.g., simultaneously or substantially simultaneously) orremoved.

Referring now to FIG. 9, the present invention may be implemented inhard disk drive 900 or any suitable device including a hard disk drive.The present invention may implement either or both signal processingand/or control circuitry, which are generally identified in FIG. 9 at902. In some implementations, signal processing and/or control circuitry902 and/or other circuits (not shown) in HDD 900 may process data,perform coding and/or encryption, perform calculations, and/or formatdata that is output to and/or received from a magnetic storage medium906 or any other suitable read channel. Correlation detection circuitry904, which may be in communication with signal processing and/or controlcircuitry 902, is configured to detect sector sync marks on magneticstorage medium 906 using correlation detection, as described in moredetail above in process 800 (FIG. 8).

Although correlation detection circuitry 904 is shown separate fromsignal processing and/or control circuitry 902, in practice these twocomponents may be integrated or combined into a single device orcomponent, if desired. Correlation detection circuitry 904 may includeat least one PLL or similar circuitry to establish at least one timingphase based on a sector preamble read from magnetic storage medium 906.Correlation detection circuitry 904 may also include any number ofaccumulators and suitable logic blocks for implementing a correlationdetector and/or a Viterbi detector. Correlation detection may then beperformed using the established timing parameters to detect sync markson magnetic storage medium 906.

In some embodiments, correlation detection circuitry 904 may beconfigured to selectively use both correlation detection and traditionalViterbi-based detection. In these embodiments, a detection type controlsignal 911 may be asserted when correlation detection is to be used todetect disk sync marks, and the detection type control signal may bedeasserted when Viterbi-based detection is to be used to detect disksync marks. The sync mark detection type (i.e., Viterbi-based orcorrelation-based) may be dynamically altered on-the-fly, if desired, byreading the detection type control signal before each sector preambleread.

HDD 900 may communicate with a host device (not shown) such as acomputer, mobile computing devices such as personal digital assistants,cellular phones, media or MP3 players and the like, and/or other devicesvia one or more wired or wireless communication links 908. HDD 900 maybe connected to memory 909 such as random access memory (RAM), lowlatency nonvolatile memory such as flash memory, read only memory (ROM)and/or other suitable electronic data storage.

It will be understood that the foregoing is only illustrative of theprinciples of the invention, and that various modifications can be madeby those skilled in the art without departing from the scope and spiritof the invention.

1. An apparatus for detecting a disk sync mark, the apparatuscomprising: a read channel to read a plurality of sector preamble bits;control circuitry to establish at least one timing phase based on thesector preamble bits; and correlation detection circuitry to: select acandidate data pattern for the disk sync mark from a plurality ofcandidate data patterns wherein each of the plurality of candidate datapattern comprises a plurality of bits in either an even or odd bitposition, and wherein selecting the candidate data pattern comprisesselecting a pattern with each bit in an odd position taking the samevalue as its preceding bit in an even position; and detect the disk syncmark using correlation detection, wherein the correlation detection isperformed at a period determined at least in part by the at least onetiming phase.
 2. The apparatus of claim 1 wherein the correlationdetection circuitry is configured to receive an indication of the periodvia a signal from the control circuitry.
 3. The apparatus of claim 1wherein the at least one timing phase comprises the phase of a sectorpreamble readback waveform.
 4. The apparatus of claim 3 wherein theperiod is equal to the period of the sector preamble readback waveform.5. The apparatus of claim 1 wherein the at least one timing phasecomprises the phase of a sector preamble readback waveform shifted by180 degrees.
 6. The apparatus of claim 5 wherein the period is equal toone-half the period of the sector preamble readback waveform.
 7. Theapparatus of claim 1 further comprising a detection type control signal,wherein the correlation detection circuitry is configured to selectivelyperform either the correlation detection or Viterbi-based detectionbased on the value of the detection type control signal.
 8. Theapparatus of claim 1 wherein the read channel comprises a magneticstorage medium.
 9. An apparatus for detecting a disk sync mark, theapparatus comprising: means for searching a candidate data pattern bitspace for a plurality of potential candidate data patterns for the disksync mark, wherein one of the plurality of potential candidate datapatterns is N-bits long, where N is a positive integer; means fordefining a plurality of right-shifted versions of each one of theplurality of potential candidate data patterns for the disk sync mark,the means for defining the plurality of right-shifted versions of eachone of the plurality of potential candidate data patterns comprisingmeans for defining N/X different right-shifted versions, eachright-shifted version shifted by a multiple of X bits; means forcomputing a minimum Euclidean distance between, respectively, each oneof the plurality of potential candidate data patterns, and the pluralityof right-shifted versions of each one of the plurality of potentialcandidate data patterns; and means for selecting, from the plurality ofpotential candidate data patterns for the disk sync mark, the candidatedata pattern with a largest minimum Euclidean distance between thecandidate data pattern and the plurality of right-shifted versions ofthe candidate data pattern.
 10. The apparatus of claim 9 furthercomprising means for writing the selected candidate data pattern withthe largest minimum Euclidean distance to a magnetic storage medium asthe disk sync mark.
 11. The apparatus of claim 10 wherein the magneticstorage medium is part of a perpendicular recording channel.
 12. Theapparatus of claim 9 wherein the means for defining the plurality ofright-shifted versions of each one of the plurality of potentialcandidate data patterns comprises means for shifting in sector preamblebits into the higher-order bit positions of the plurality ofright-shifted versions.
 13. The apparatus of claim 9, wherein: each ofthe plurality of candidate data patterns comprises a plurality of bitsin either an even or odd bit position; and the means for searching foreach one of the plurality of potential candidate data patterns for thedisk sync mark comprises means for selecting a pattern with each bit inan odd position taking the same value as its preceding bit in an evenposition.
 14. The apparatus of claim 9 wherein the means for searchingthe candidate data pattern bit space for the plurality of potentialcandidate data patterns comprises means for searching the bit space overa single target polynomial.
 15. The apparatus of claim 9 wherein themeans for searching the candidate data pattern bit space for theplurality of potential candidate data patterns comprises means forsearching the bit space over a plurality of common target polynomials.16. The apparatus of claim 9 further comprising means for defining aplurality of right-shifted versions of a sign-flipped one of theplurality of potential candidate data patterns.
 17. The apparatus ofclaim 16 further comprising: means for computing the minimum Euclideandistance between one of the plurality of potential candidate datapatterns and the plurality of right-shifted versions of the sign-flippedone of the plurality of potential candidate data patterns; and means forselecting the candidate data pattern with the largest minimum Euclideandistance between the candidate data pattern and both the plurality ofright-shifted versions of the candidate data pattern and the pluralityof right-shifted versions of the sign-flipped candidate data pattern.18. The apparatus of claim 17 further comprising means for determining apolarity of the disk based on the selected candidate data pattern withthe largest minimum Euclidean distance.
 19. A method for selecting adisk sync mark, the method comprising: searching a candidate datapattern bit space for a plurality of potential candidate data patternsfor the disk sync mark, wherein one of the plurality of potentialcandidate data patterns is N-bits long, where N is a positive integer;defining a plurality of right-shifted versions of each one of theplurality of potential candidate data patterns for the disk sync mark,wherein defining the plurality of right-shifted versions of each one ofthe plurality of potential candidate data patterns comprises definingN/X different right-shifted versions, each right-shifted version shiftedby a multiple of X bits; computing a minimum Euclidean distance between,respectively, each one of the plurality of potential candidate datapatterns, and the plurality of right-shifted versions of each one of theplurality of potential candidate data patterns; and selecting, from theplurality of potential candidate data patterns for the disk sync mark,the candidate data pattern with a largest minimum Euclidean distancebetween the candidate data pattern and the plurality of right-shiftedversions of the candidate data pattern.
 20. The method of claim 19further comprising writing the selected candidate data pattern with thelargest minimum Euclidean distance to a magnetic storage medium as thedisk sync mark.
 21. The method of claim 20 wherein the magnetic storagemedium is part of a perpendicular recording channel.
 22. The method ofclaim 19 wherein defining the plurality of right-shifted versions ofeach one of the plurality of potential candidate data patterns comprisesshifting in sector preamble bits into the higher-order bit positions ofthe plurality of right-shifted versions.
 23. The method of claim 19,wherein: each of the plurality of candidate data patterns comprises aplurality of bits in either an even or odd bit position; and searchingfor each one of the plurality of potential candidate data patterns forthe disk sync mark comprises selecting a pattern with each bit in an oddposition taking the same value as its preceding bit in an even position.24. The method of claim 19 wherein searching the candidate data patternbit space for the plurality of potential candidate data patternscomprises searching the bit space over a single target polynomial. 25.The method of claim 19 wherein searching the candidate data pattern bitspace for the plurality of potential candidate data patterns comprisessearching the bit space over a plurality of common target polynomials.26. The method of claim 19 further comprising defining a plurality ofright-shifted versions of a sign-flipped one of the plurality ofpotential candidate data patterns.
 27. The method of claim 26 furthercomprising: computing the minimum Euclidean distance between one of theplurality of potential candidate data patterns and the plurality ofright-shifted versions of the sign-flipped one of the plurality ofpotential candidate data patterns; and selecting a candidate datapattern with a largest minimum Euclidean distance between the candidatedata pattern and both the plurality of right-shifted versions of thecandidate data pattern and the plurality of right-shifted versions ofthe sign-flipped candidate data pattern.
 28. The method of claim 27further comprising determining a polarity of the disk based on theselected candidate data pattern with the largest minimum Euclideandistance.